The photos are blurry so it's hard to determine.
First thoughts is if found on the R/R tracks it is automatically property of the said R/R, and it's classified as a theft. ( just messing with you-but that is true)
If I picked it up I would think it was one of the following- Sperry Train grinding residue
The grinding residue is built up as would on a bench grinder and it drops off along the rails.